Stabilization of positive sol refractory grain slurries

ABSTRACT

SLURRIES OF REFRACTORY MATERIAL SUCH AS MOLOCHITE SUSPENDED IN AQUEOUS SOLS OF POSITIVELY CHARGED ALUMINA COATED COLLOIDAL SILICA PARTICLES ARE STABILIZED BY THE ADDITION OF CERTAIN ACIDS, PARTICULARLY HYDROXY SUBSTITUTED ORGANIC ACIDS SUCH AS HYDROXYACETIC ACID.

United States Patent ()1 lice 3,764,355 STABILIZATION OF POSITIVESOL-REFRACTORY GRAIN SLURRIES Earl P. Moore, Jr., Wilmington, Del.,assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. NoDrawing. Filed May 20, 1971, Ser. No. 145,534 Int. Cl. C04b 35/48 US.Cl. 106-57 11 Claims ABSTRACT OF THE DISCLOSURE Slurries of refractorymaterial such as molochite suspended in aqueous sols of positivelycharged alumina coated colloidal silica particles are stabilized by theaddition of certain acids, particularly hydroxy substituted organicacids such as hydroxyacetic acid.

BACKGROUND OF THE INVENTION Slurries of refractory material suspended inaqueous sols of positively charged silica particles coated with aluminaare useful in a rapid process for making precision investment castingmolds and other ceramic laminates. This process is described in detailin commonly assigned application Ser. No. 49,916. Briefly, laminates arebuilt up on a pattern by alternately dipping the pattern in reagentscontaining oppositely charged materials such as a sol of positivelycharged particles followed by a sol of negatively charged particles. Toincrease the rate of buildup and impart the desired properties to theceramic article, refractory material is slurried in the reagentcontaining the charged particles. Slurries of refractory grain andrefractory fibrous material suspended in aqueous sols of positivelycharged alumina coated colloidal silica are particularly useful in thisprocess. However, these slurries tend to thicken on standing and theirmaximum working life is only about six to eight days. This inventionprovides stabilizing agents with which the working life of theseslurries can be extended to 25 days or more.

SUMMARY OF THE INVENTION The working life of slurries of up to 80% byweight particulate or fibrous refractory material suspended in aqueoussols of positively charged alumina coated colloidal silica can beextended by the addition of a stabilizer. Stabilizers useful forextending the life of these slurries are water solublehydroxy-substituted aliphatic and aromatic carboxylic acids as Well assome mineral acids such as phosphoric acid and certain other organicacids such as ethylenediaminetetraacetic acid and nitrilotriacetic acid.Addition of from 0.00834 to 0.0834 part by weight of stabilizer per partof colloidal solids provides the improved stability in the slurries ofthis invention.

In particular, this invention is directed to the use of stabilizingagents such as hydroxyacetic acid, citric acid, maleic acid, tartaricacid, glnconic acid, diglycolic acid, phosphoric acid,ethylenediaminetetraacetic acid and nitrilotriacetic acid. The last twostabilizing agents named, polyfunctional carboxylic acids, must bepartially converted to salts to adjust their water solubility so thatuseful amounts may be dissolved in the slurries of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventtion will now bedescribed in detail with particular reference to the elements of thestabilized slurries of this invention.

Positive sols This invention utilizes acidic sols of positively chargedcolloidal particles which are composed of a silica core coated withalumina. Such sols are referred to herein as positive sols. Sols of thistype are described in US. Pat. 3,007,878.

An example of a charged alumina-coated silica sol which is particularlyuseful in this invention is one in which there is one mole of aluminumper mole of surface silica and which is prepared by a process describedas follows:

264 lbs. of Ludox HS colloidal silica containing 40% SiO by weight, thesilica particles having an average particle size of 1215 millimicronsand a specific surface area of about 215 m. /g. SiO is adjusted to pH7.50 with 821 grams of a 1:1 mixture of concentrated hydrochloric acidand water. The sol is mixed with 62.8 lbs. of 50% chlorohydrol (Al (OH)Cl) and 61.7 lbs. of water by introducing it at a rate of 25 lbs/minuteinto a centrifugal pump circulating the basic aluminum chloridesolution. The clear fluid intermediate product is heated to 60 C. inone-half hour and at 60 C. for two hours, cooled to 20 C., and stirredwith a Lightnin mixer as Well as circulated with the pump as 600 gramsmagnesium hydroxide dispersed in 1800 grams Water is introduced in 5minutes to bring the pH to 5.65. Agitation and circulation are continuedfor 2 hours. The clear stable product contains 26.4% SiO 4.2% A1 0 1.5%Cl and 0.23% MgO. The mole ratio of aluminum to surface SiO is 1:1. ThepH of the product after several weeks ageing is 4.60, the viscosity is16 cps, and the specific gravity at 25 C. is 1.23. This product(referred to hereinafter as Positive Sol 130M), is the positive solwhich is used in the examples set forth hereinbelow.

Positive Sol 130M is stabilized by chloride ion. As described in US.3,007,878 other anions, such as formate, acetate, lactate, nitrate,bromide, perchlorate, bromate, and trichloroacetate, can be used insteadof chloride.

Refractory material Several types of refractory material may be used inthe slurries of this invention. Any finely divided particulate orfibrous refractory material may be used provided that it does not reactwith the positive sol. Among the suitable granular refractory materialsare zircon, aluminosilicates such as mullite, sillimanite and molochite,fused silica and alumina. Useful refractory fibers are Kaowool, volcanicrock fibers, glass fibers, and Fiberfrax, aluminosilicate fibers.Slurries of these refractory materials are prepared as described in US.patent application Ser. No. 49,916. The slurries of this inventioncontain from up to by weight of particulate refractory material based onthe total weight of the slurry.

Stabilizing agents The stabilizing agents useful for prolonging theWorking life of the slurries of this invention are generallyhydroxy-substituted aliphatic and aromatic carboxylic acids as well asother acids such as phosphoric acid, ethylenediaminetetraacetie acid andnitrilotriacetic acid. The apparent chemical function of the stabilizingagents is to complex with ionic impurities introduced into the slurrywith the refractory material which have a destabilizing effect on thepositive sol. The use of the stabilizing agents in the particulaterefractory-positive sol slurries extends their working life from 68 daysto as much as 25 days or more. Without addition of these stabilizingagents the slurries tend to thicken and become unuseable in about 68days. The stabilizing agents most useful in this invention arehydroxyacetic acid, citric acid, maleic acid, tartaric acid, gluconicacid, diglycolic acid, phosphoric acid, ethylenediarninetetraacetic acidand nitrilotriacetic acid. These last two stabilizing agents requirepartial conversion to the salt of the orgaic acid in order to reach asolubility level adequate for use in the slurries of this invention. Forexample, the nitrilotriacetc acid should be adjusted to pH ofapproximately 3.3 with sodium hydroxide. The ethylenediaminetetraaceticacid is best used as the disodium salt. The most preferred stabilizingagents 0.0834 parts by weight of said stabilizer per part of colloidalparticles.

3. The slurry of claim 2 containing from 0.0167 to 0.050 parts by weightof said stabilizer per part of colare hydroxyacetic acid and citricacid. Mixtures of these loidal Particles two Stabilizing i i arePrefeFed- 4. The slurry of claim 1 wherein the stabilizer is by- Theuseful l1m1ts of stabilizing agents in the slurry of droxyacatic aciithis mventlon appear to be "i :9 a gram per 5. The slurry of claim 1wherein the stabilizer is 100 grams of a 30% colloidal solids pos1t1vesol at the citric acid lower limit and approximately ten times as much,2 /2 6. The slurry of claim 1 wherein the stabilrzer 1s a grams per 100grams of 30% SOlldS positive sol at the t f h d o add d it upper limit.This translates to from 0.00834 to 0.0834 g I t am parts by weight ofthe stabilizer per part of colloidal T e P o c mm w 3mm the Sta solids.The preferred stabilizer range is from 0.017 to 0.05 hydroxyacetlc pansby Weight of stabilizer per part of coloidal Solids The slurry of claim2 wherein the stabilizer 1s cltric Not all of these stabilizing agentsare effective over the acldentire range set out. Some of the acids causepositive sol The 0"? clalm 2 whsl'em f tab111ler 1S a to gel at thehigher concentrations. However, all of the mlxture of 011119 acld 1l'lydroxyiilcetlc acldstabilizing agents are operative within the rangedescribed A PFOWSS for slablllllllg Slur I 16s of from p 30% and theoptimum level for any particular agent is easily particulate refractorymaterial suspended in an aqueous determined. Some of the stabilizers canbe used at even sol of positively charged colloidal silica particlescoated higher concentrations with no ill effect, but no advantage withalumina comprising adding from 0.00834 to 0.0834 is obtained thereby.parts by weight of a stabilizer per part of colloidal parti- Thefollowing examples further illustrate this invention. 2 cles, saidstabilizer being a complexing agent selected 5 from the group consistingof hydroxyacetic acid, citric EXAMPLES acid, maleic acid, tartaric acid,gluconic acid, diglycolic In the following examples the indicatedstabilizing acid, ethylenediaminetetraacetic acid and nitrilotriaceticagents were added in the amounts shown to a slurry of acid. 1250 g.Positive Sol 130M solids) and 2150 g. 11. The slurry of claim 3 whereinthe stabilizer is molochite 200 mesh refractory grain. 30 hydroxyaceticacid, citric acid or a mixture thereof.

Amount added Parts/part Slurry Final Example colloidal pH of N 0.Stabilizing agent Gram solids days slurry 6-8 4.2-4.5 4. 375 0. 01157 54. 15 8.75 0. 0233 7 4.0 17.5 0. 0455 24 3.85 a. 125 0. 00884 8 4. s5 5.25 o. 0157 12 4.15 0.375 0.025 24 4.03 3. 1259. 375 0. 00534-0. 025 24a. 90 5. 25-5. 25 0. 0157-0. 0107 20 3. 72 5 25 1.375 0. 0157+). 025 274. 05 5. 25 0. 0157 9 4. 24 5.25 0.0157 9 4.15 Gluconic acid 12.5 0.03312 4.21 Diglyeolic acid 12.5 0.033 14 3.75 Phosphoric acid 8.125 0.021611 4.25 Ethylenediaminetetraacetic acid-disodium- 6. 25 0.0167 10 4. 5217 Nitrilotn'acetic acid (pH adjusted to 3.3) 6.25 0. 0167 14 4. 65

What is claimed is: References Cited 1. A slurry of up to 80% by weightrefractory material UNITED STATES PATENTS suspended in an aqueous sol.of positively charged colloidal silica particles coated with aluminacontaining a 66 p i 252-313 R 966 Mmdrck et a1 252-313 S stabilizer forextending the Workng life of the slurry, said 3 445 250 5/1969 Preece106 69 stabilizer being a complexing agent selected from the 35O79444/1970 Moo I6 106-57 group conslstlng of hydroxyacehc acld, c1tr1c acid,maleic acid, tartaric acid, gluconic acid, diglycolic acid, ethylene-JAMES POER, i ary Examiner diaminetetraacetic acid and nitrilotriaceticacid. U S C1 X R 2. The slurry of claim 1 containing from 0.00834 to

